Security device comprising a stop member for drilling instrument used in particular in dental surgery and device pre-calibrating and storing drilling depth

ABSTRACT

A security device includes a stop member for a rotary drilling instrument used in particular in dental surgery. The stop member is capable of being positioned at a desired depth of drilling and of preventing the depth from being exceeded. The two elements drilling instrument and the stop member include coupling elements directly or indirectly linking them together and co-operating elements capable of transforming their movement, such that when the device is operating, the drilling instrument is retracted rearwards relative to the drilling direction and to the position of the stop member under the action of the elements when the front part of the stop member comes in contact with a neighboring surface. The co-operating elements preferably consist of a screw-nut system, the direction of the thread(s) of the system and the rotating direction of the drill being opposite. A device comprising a gauge containing a piston enables to pre-calibrate and store the desired drilling depth.

BACKGROUND OF THE INVENTION

The invention relates to a safety device comprising a stop for adrilling instrument of use inter alia in dental surgery. The inventionalso relates to a device for precalibrating and storing the depth ofdrilling and directly related to an advantageous process of using thesafety device.

One preferred application of these devices is to dental implantology.

As is known, during some operations, inter alia the treatment of rootcanals, some dental lesions, hard dental or maxillary tissue (e.g. whenfitting a dental implant) the depth of penetration, drilling or boringby a specific instrument (endodontic instrument, dental bur or drill,implantology drill bit, etc) must be to a clearly defined depthdetermined by the practitioner as required by the patient.

The defined depth must be actually obtained but not exceeded. Thiscondition is particularly important in dental implantology, where it isessential to ensure the long-term reliability of the infrastructure forinserting into the jawbone in order to hold a fixed dental prosthesis,but without any risk of injury to neighbouring structures.

For these reasons, manufacturers propose various devices for conformingto these constraints.

For example, in a very simple but very frequently-used device in dentalimplantology, the instrument has markings in the form of grooves,coloured or otherwise, at different distances P₁, P₂, . . . P_(n) on theactive part, i.e. the cutting part of the drill bit, so that thedrilling operation can be stopped at the right moment, when thepractitioner sees that the previously-determined depth has been reached(visual marks, reference 14 in the accompanying FIG. 1).

In other more sophisticated devices (which will be briefly describedhereinafter) the instruments are provided with stops, although it shouldbe noted that not one of these prior-art devices is suitable for dentalimplantology nor intended for use in this special sector.

EP-0 515 274 describes an instrument (a helical drill bit) used indental surgery and having a cutting part 7 formed with transversegrooves which can serve as an optical reference part indicating thedepth of drilling. The practitioner can place a stop ring (8, 9) on theappropriate groove, i.e. the groove corresponding to the depth of thehole which he intends to make, so that he will be better able to checkthe depth of drilling by positioning the stop, which will thus remainfixed during the required operation.

The teaching in FR-2 613 212 is similar except that the non-operativepart 1 b has a number of peripheral transverse notches for positioning aclip constituting a fixed depth stop for determining and checking thelength of that part of the instrument which is to penetrate into thetooth.

In a variant disclosed in U.S. Pat. No. 4,526,542, the stop is screwedto the drill shank until locked against the rear surface of the cuttingpart, the screwthread or internal screwthread being oriented oppositelyto the rotation of the drill, so that the stop serves as a lock-nut.When the stop comes in contact with a neighbouring surface it will tendto be locked more tightly on to the shoulder presented by the workingpart at the top. As we shall see hereinafter, this is contrary to theaim of the inventor of the present invention.

The International Application WO 98/03125 discloses a drill bit having aworking part formed with calibrated recesses spaced apart (relative tothe drill axis) by a millimeter in each case. An annular stop can bemoved along the said working part and can be immobilised at each recess,when a pin disposed in the stop can engage in a recess under the actionof a spring.

These devices can display the depth of drilling or boring intended bythe practitioner, i.e. the stop on the instrument can be positioned independence on the said depth, and also serves as a safety element sincethe aim, once the instrument has been adjusted, is to ensure that thesaid depth is not exceeded during the operation.

All these known stop devices, however, have at least three kinds ofdisadvantages as regards the specialist in dental implantology whowishes to drill for the purpose of fitting implants. Firstly, insofar asa stop is simply fixed at a predetermined working position, the priorart devices to not always make optimum use of the depth of the site ofoperation, apart from the fact that the accuracy of depth display isunsatisfactory. Secondly, in cases where the working conditions are notoptimum, these devices may produce an oval hole and thus endanger theprimary stability of the implant, in which case a satisfactory resultcannot be guaranteed, particularly as regards the stability of theimplant (see hereinafter). Finally the prior art devices are uneconomic.

Although these three groups of disadvantages can easily be understood byexamining the prior publications, it is worth explaining them further:

When the instrument, which by definition is subjected to pressure duringoperation, reaches the defined drilling depth, the stop may slip onmeeting a surface inclined with respect to the axis of the instrument oran irregular surface, concave or convex at places (which is veryfrequently the case in implantology, where dental implants have to befitted on to an apex of a bone crest only slightly larger in diameterthan the implant). The probability of slipping is far from negligible,and is greater in cases where the bone for drilling is very spongy andloose or in the very frequent cases where the uppermost part of the apexof the bone crest (always on the same side as the tongue) is of hardcortical bone whereas the lower part (on the buccal side) is softer(only slightly corticalised). The result of such an eventuality willinevitably be a side slip, i.e. a lateral drift of the drill axis, and aconsequently oval hole.

This serious disadvantage of the known devices is one reason for lack ofprimary stability of the implant.

Techniques and knowledge in implantology are in process of development.One development is towards use of non-traumatic techniques, whenpreparing holes in bones for receiving “screw implants in root form”.Another development is towards adapting drilling techniques to the fourbone densities (type D1 comparable to oak wood down to type D4,corresponding to expanded polystyrene) normally found in dentalimplantology. Consequently the dentist, or more precisely theimplantologist, will need a very large number of preferably increasingdrill diameters (usually from 1.5 to 5 mm). This is to prevent injurythrough heating of the dense bone by too rapid drilling of the hole.Another purpose is to adapt the preparatory work to the differentdiameters of the implants at present available. A final aim, in the caseof spongy or loose bone, is to produce holes sufficiently tight forinserting an implant with good primary stability without risk offracturing the bone trabiculation when inserting an implant, throughinsufficient drilling of the hole in the bone.

These situations are not taken into account by any of the known safetydevices for limiting the depth of drilling.

Finally no citation discloses means for precalibrating the planned depthof drilling and storing the said depth.

SUMMARY OF THE INVENTION

The object of the invention is to reduce the said disadvantages and takeaccount of the considerations stated.

The object is achieved by the device and method defined in the claims.

The claimed stop, in addition to being mobile, can be described as“active” in contrast to the “passive” character of the stops known inthe prior art, in that the co-operation as described between the stopand the drilling tool or instrument inevitably and automatically resultsin withdrawal of the instrument from the hole just made by thepractitioner, as soon as the desired depth is reached. The moving stoppositioned according to the invention effectively prevents the presetdrilling depth from being exceeded, together with any other risks suchas ovalisation or lesions.

To sum up, as soon as the stop meets an obstacle and is thus preventedfrom rotating, the stop moves exclusively in the direction forshortening the length of the exposed working part of the instrument,thus avoiding any accident.

Likewise if, when the instrument is set in rotation, the inertia of thestop causes it to rotate on the co-operating means (the screw and nutsystem in one example) connecting these two elements (i.e. the stop andthe instrument, more particularly a drill or drill bit), the resultingrotation communicated to the stop will involve axial displacementthereof in a direction such that the displacement will shorten theportion of the working part of the tool initially projecting from thestop. This is a safety factor, particularly in dental implantology.

The automatic withdrawal of the drill bit at the end of traveleliminates any risk of drift by the tool, i.e. any risk of ovalisationof the hole due to side-slipping of the stop. This feature, as we haveseen, is of fundamental importance in dental implantology, particularlyin the case of slightly corticalised bone.

Retraction of the drill bit also prevents any excess drilling during thepreparation of a hole in very spongy bone (usually in the posterior partof the jaw bones, particularly the upper jawbone). It is essential toavoid any excessive drilling of the tender bone. The lack of accuracyresulting from excessive drilling will destroy the primary stability ofthe implant, such stability being essential for incorporation in thebone (i.e. the creation of a stable biological link between the bone andthe implant so that it can bear loads efficiently and over a longerperiod without clinical symptoms. This is a fundamental principle andaim of modern implantology).

The adjustment of the depth of drilling by the prior-art stop devices,using bearings or notches, is rudimentary and insufficiently accurate.By contrast, in the device according to the invention, the stop isconnected to the non-operative part (shank) of the instrument,preferably via a micro-screwthread, permitting easy, fine and accuratemicrometric adjustment of the depth of drilling.

The advantage of micrometric adjustment, by precalibrating the depth ofdrilling, using an X-ray or tomographic film to scale 1:1, is thatoptimum use is made of the depth of bone available, without risk ofinjury to neighbouring structures. The chosen depth of drilling can beadjusted easily during the operation, by very accurate micrometricmovement of the stop, leaving the drill in the bone hole under X-raymonitoring. In the case of digital radiography, the depth can be checkedand corrected if required (i.e. the stop can be re-positioned)immediately, with the additional advantage of better use of time.Finally the micro-screwthread is a means of accurately correcting andcompensating the error due to prestress of the edge of the stop by thenearest part of the bone surface (the position of a stop is pre-adjustedrelative to the drill axis—an impossible operation in the case of a stopwith bearings or notches).

This method also has an economic advantage, since the standard diameterof shanks (for different diameters of the working part) of drills usedin implantology is always 2.2 millimeters, so that a standard stop canbe used for a whole range of drills. In other applications wherenecessary, the standard stop can be supplemented by stops adapted toother shank diameters. In all cases, the device according to theinvention enables the number of stops to be narrowly limited, perhaps toa single standard type covering practically all sizes of drill (afeature which makes these devices all the more advantageous in sectorssuch as implantology where wear is particularly important).

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the device according to the invention will now bedescribed in detail by way of non-limitative example with reference tothe accompanying drawings in which:

FIG. 1 represents a drill bit having a threaded shank;

FIG. 2 shows an embodiment of a movable stop;

FIGS. 3 and 4 show the safety device when assembled, with the stop intwo different positions;

FIG. 5 is a larger-scale view of another embodiment of a movable stop;

FIG. 6A shows a conventional drill;

FIG. 6B, on a larger scale, shows an intermediate element for connectionand co-operation between the drill and the stop, in one embodiment;

FIG. 6C is a section along line VI C-VI in FIG. 6B;

FIG. 7 shows the device in the working phase (end of phase when the stopcomes against a diagrammatically indicated wall), and

FIGS. 8, 9 and 10 show the device for precalibrating and storing thedepth of drilling.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An instrument 10, in the present case a drill bit used more particularlyin dental surgery (see FIG. 1) has an active or cutting part 11 (i.e.the working part) provided along its length with conventional helicalcutting edges (directed towards the right though not shown on thedrawing) and with marks or grooves 14 likewise conventional on this kindof drill, with a shank 12 adapted to engage in a chuck or manualcomponent (not shown) for driving the drill in rotation. The rotation isusually directed to the right viewed in the direction from shank toactive part (see arrow R in FIG. 1), the edges on the active part beingoriented in the same direction. In its front part, the shank 12 has ascrewthread over a distance D such that all the usual depths of drillingcan be displayed by a stop 20 (see hereinafter). The screwthread 15 isin the opposite direction to the direction of rotation R. In the exampleshown, therefore, the screwthread is left-handed. It is advantageous tochoose a micro-thread or at least a very fine thread for the purpose ofmicrometric adjustment of the desired drilling depth, as will be seenhereinafter.

The component 20 shown in FIG. 2 is a movable stop formed at its rear 22with an internal screwthread 24 corresponding to the screwthread 15 onthe drill 10, the components 22, 24 forming the equivalent of a nut(general reference 25). The nut 25 extends towards the front of atubular body 21 which partly covers the working part 11. In thisembodiment, therefore, the stop 20 has a basically bell shape. The innerdiameter of the body 21 is made such that the stop can be fitted ondrills of different diameter (the most frequently used varying between1.5 mm and 5 mm approx.). A standard stop or, if required, two standardstops can thus be designed, one for drills having a working part between1.5 mm and 3.5 mm in diameter and the other for drills having a workingpart between 2.5 mm and 5 mm in diameter. These two stops will besufficient for a device of universal application, since furthermore theshank diameter of most drills used in implantology is uniformly equal to2.2 mm.

The stop 20 co-operates with the drill 10 mounted on it, like a nut on ascrew (screw and nut system 15-25 accompanying or converting movement inrotation into movement in translation and vice versa, more particularlyin the sense that when the rotating nut, i.e. the stop 20, isimmobilised the screw, i.e. the drill bit 10, continues to rotate butmoves longitudinally). The stop 20 can then be finely adjusted by thepractitioner, so that the exposed length 16 of the working part 11between the front surface 23 of the stop and the end part 13 of theworking part of the drill 10 will correspond very accurately to adesired depth P for the planned drilling (FIG. 3).

The means of co-operation between the drill 10 and the stop 20, i.e. thescrew and nut system 15 and 25 in the example, must be designed so thatthe frictional forces encountered are just sufficient for the drill 10,when driven in rotation, to entrain the stop 20 without the stop movingin linear manner (the result of a difference between the angularvelocity of the stop and that of the drill (relative motion)). Thepreviously-mentioned choice of the nature and kind of the screwthreadpreferred for this system (micrometric thread), in addition toaccurately positioning the stop, serves or at least contributes tosatisfying this condition, since the frictional forces must be such thatthe withdrawal of the drill begins as soon as the stop is immobilised,because the drill comes in contact with neighbouring tissues. Dependingon the case, one and/or the other component of the said co-operatingmeans, i.e. the internal thread 24 and/or the thread 15, can be providedwith a braking means which increases the frictional forces. Anothervariant would be to fit a ring or O-ring 27 (FIGS. 3 and 4) against therear surface 26 of the nut 25 and on the screwthread 15. Of course, anyother means known to the skilled man may be used. By way of example, inanother variant, the components 15 or 25 can be magnetised or providedwith a magnetic chip, to obtain a preset frictional force between thesaid components.

During the operation, when the planned depth of drilling is reached, thefront surface 23 of the stop 20 comes against the neighbouring surface 9(see FIG. 7). The resulting bearing force on the stop 20, as soon as itequals the frictional forces on the connection between the drill 10 andthe stop 20, will immobilise the stop 20 and thus retract the drill bitinto the tubular body 21 (see FIG. 4, where the portion 16 (equal to P)of the working part 11 (see FIG. 3) has changed to a value P′ (FIG. 4)with P′<P, as a result of the immobilisation in conjunction with therotation of the drill bit).

This reduction is also indicated by the marks 14 on the drill bit ifprovided thereon (see hereinbefore, introductory part).

The shape and structure of the stop can of course vary. By way ofexample, FIG. 5 shows a stop 30 with a basket structure. A rearinternally threaded part 32, 34 forms a nut 35 and is prolonged by acylindrical cage 31 comprising a bottom hoop 38 (front surface 33)connected to the nut 35 by four bars 37. This structure does notcompletely cover the working part 11 of the bit 10, i.e. it leaves thispart exposed so that the bit is more efficiently cooled, since a largersurface area of the working part will then be in contact with thecooling liquid. In other respects, this stop of course serves the samepurpose as the stop 20; the nut 35 co-operates with the screw 15 on thebit 10, which can be withdrawn into the cage 31 as soon as the frontpart 33 comes against the neighbouring surface (reference 9 in FIG. 7).

In a variant embodiment (in which the drill bit and the stop areconnected indirectly), the drill bit 10A (see FIG. 6A) is identical inall respects with the bit 10 in FIGS. 1 to 4 except that it has noscrewthread 15 (or any other means for converting rotation into axialmovement) on the part 12, but only has a hollow 17 at a defined place onthe said part. A removable intermediate component, general reference 40(see FIGS. 6B and 6C, the latter being a section along the line VI C inFIG. 6B) is also provided. The component 40 has the general shape of asleeve 41 having a given length equivalent to the length D (compareFIG. 1) whereas the diameter of the internal thread 42 is equal to thediameter of the shank 12 (which by definition is not threaded in thisvariant embodiment). The sleeve 40 has a left-hand screwthread 45. It isadapted to be accurately fitted on the shank 12 at the required place(advantageously corresponding to the position of the screwthread 15 onthe bit 10) and to be locked thereon by a locking micro-screw 44 (FIG.6C) inserted into a screwthreaded hole 43. The end (no reference number)of the locking micro-screw can engage in the hollow 17 in the shank 12(FIG. 6A; a hollow of this kind can of course easily be made by thepractitioner himself on site if the drill shank does not have such ahole). As can be seen, since the component 40 is movable, the device hasincreased, unrivalled flexibility and usefulness. More particularly,families of sleeves can be provided, all having the same outer diameterand the same screwthread but different internal diameters (depending onthe diameters of the drill shanks). For example a single stop 20A, 30Ahaving a body 21A, 31A and a front surface 23A, 33A (see FIGS. 2 and 5)with a nut 25A, 35A formed by the rear part 22A, 32A and having aninternal screwthread 24A, 34A corresponding to the screwthread 45, canbe used for an entire range of drill bits.

In general, of course, the direct or indirect co-operation between thestop and drill shank can be obtained by any other known means equivalentto those described by way of example, i.e. adapted to convert movementin translation into movement in rotation and vice versa (e.g. one ormore helical grooves and balls).

In addition to the accuracy of operation by the device according to theinvention, any risk of exceeding the previously-determined depth and anyconsequent risk of ovalisation of the hole and injury to the tissues arecompletely excluded.

Owing to its flexibility and general usefulness, the safety deviceaccording to the invention also has a considerable financial advantageover the known fixed-stop safety devices. As already stated, it will besufficient for the specialised practitioner to obtain a single kind ofstop (standard stop) e.g. in dental implantology, where the diameter ofdrill bits is uniformly 2.2 mm. In other cases, the user will need onlya very limited number of different stops, depending on the application.In all cases there will be no need to keep a stock of drills with fixedstops corresponding to different depths.

Also the bodies 21; 21A; 31; 31A of the stops 20; 20A; 31; 31A can bemade of any sterilisable opaque or transparent material.

As can be seen, the chosen micrometric screwthread enables the defineddrilling depth to be very finely adjusted. On the one hand the distance,i.e. the depth between the end point of the bit and the front surface ofthe stop, can be measured in various ways. On the other hand, it may beadvantageous to pre-calibrate the detailed depths of drilling for anumber of implants and store them during an operation. To this end, aset of specially designed gauges are provided and can be disposed in asuitable block or box and used in a process described hereinafter. Eachgauge 50 (see FIGS. 8, 9 and 10) comprises a cylinder 51 graduated inmillimeters (e.g. from zero to 30 mm), made of transparent materialsterilisable in an autoclave or a chemiclave, containing a piston 55(with upper surface reference 57), preferably provided with a piston rod56 for easy resetting to zero. The 0 mm mark is flush with the topsurface 52 of the cylinder 51. The outer diameter of the cylinder 51 isapproximately equal to the outer diameter of the stop 20; 30, so thatthe front surface 23, 33 of the stop 20, 30 can come exactly against thetop part 52 of the gauge (see FIG. 10). In a first step, the uppersurface 57 of the piston 55 is opposite the zero mark (FIG. 8). In asecond step, the practitioner places the end 13 of the working part 11of the bit against the piston 55, thus driving it into the cylinder bodyuntil the piston surface 57 is level with the mark corresponding to theplanned depth of drilling (e.g. 15 mm, see FIG. 10). He then acts on thestop by rotating it until the front surface 23, 33 thereof comes againstthe edge 52 of the cylinder; the stop is in position for the planneddrilling operation, as regards the displayed depth. Depending on thesituation, i.e. when the practitioner intends to drill a number ofholes, it appears advantageous to pre-calibrate and store the choice ofthe various depths. To this end, the practitioner will use as manygauges as planned depths, a particularly interesting idea when a numberof dental implants are fixed on the same patient during the sametreatment session.

What is claimed is:
 1. A safety device comprising: a stop for a rotarydrilling instrument, the stop being movable on a rotary drillinginstrument to a position corresponding to a desired drilling depth ofthe rotary drilling instrument to prevent exceeding the desired drillingdepth; and means for converting rotary motion of the rotary drillinginstrument to relative motion between the stop and the rotary drillinginstrument to cause the drilling instrument to withdraw to the rearrelative to the direction of drilling by action of said means as soon asa front part of the stop comes in contact with a surface.
 2. The safetydevice according to claim 1, wherein the means for converting comprisesa nut and screw, the direction of threads of the nut and screw beingopposite to the direction of rotation of the drill in operation.
 3. Thesafety device according to claim 2, wherein the screw comprises ascrewthread on a shank of the rotary drilling instrument and the nutcomprises a corresponding internal thread of a rear part of the stop. 4.A The safety device according to claim 2, wherein the screw comprises anintermediate element in the form of a sleeve with a body that isexternally screw-threaded, said intermediate element being adapted to befitted on a shank of the rotary drilling instrument, and the nutcomprises a corresponding internal thread of a rear part of the stop. 5.The safety device according to claim 1, wherein the stop has a body thatextends at least partly around the drilling instrument, the desireddrilling depth corresponding to a portion of the drilling instrumentprojecting from the front part of the stop, the desired drilling depthbeing pre-adjustable by action of the converting means, and the lengthof the portion decreasing when the front part comes in contact with thesurface.
 6. The safety device according to claim 1, wherein the stopcomprises a body that is one of a tubular solid and an openwork cage. 7.The safety device according to claim 6, wherein the body is made of asterilisable, opaque or transparent material.
 8. The safety deviceaccording to claim 6, wherein the converting means comprise a brakingelement.
 9. The safety device according to claim 1, further comprising agauge in the form of a transparent graduated body and a piston, an uppersurface of the graduated body corresponding to a zero mark on a scale onthe graduated body and wherein an outer diameter of the graduated bodyis approximately equal to an outer diameter of the stop so that thefront part of the stop contacts an upper surface of the graduated bodywhen the graduated body is mated with the stop.
 10. A method ofprecalibrating and storing desired drilling depth using the safetydevice according to claim 9, comprising the steps of pressing the frontpart of the stop against the upper surface of the graduated body anddriving the piston inside the graduated body by action of the convertingmeans, the action resulting in a thrust exerted on the piston by therotary drilling instrument until an upper surface of the piston isopposite a graduation mark corresponding to the desired drilling depth.11. A safety device comprising: a drill having an operating bit at oneend thereof and an external screw thread on a shank thereof, saidexternal screw thread being wound in a first direction that is oppositeto a second direction of operation of said drill; a stop coaxial withsaid drill and having a rear end with an internal screw thread thatmates with said external screw thread of said drill and a front end thatengages a work surface during operation of the device, said stop beingmovable longitudinally along said drill by rotating said stop; and saidinternal and external screw threads having a frictional engagement thatovercomes an inertia of said stop to cause said stop to rotate in thesecond direction with said drill when said drill is rotating in thesecond direction and said front end of said stop is not engaging a worksurface, the frictional engagement being overpowered by engagement ofsaid front end of said stop with a work surface to cause said stop torotate relative to said drill and move toward said operating bit.